Terrorism has psychological and social effects. The emotional and behavioral responses to disasters occur in four phases. Psychological responses to terrorism may include acute stress disorder, posttraumatic stress disorder, and other clinical diagnosis. Studies indicate that in the year following terrorist incidents, the prevalence of PTSD in directly affected populations varies between 12% and 16% but it may also decline 25% over the course of that year. There are several vulnerability factors for developing PTSD after a trauma. As known not all subject develop PTSD symptoms after trauma. Those factors are presence of childhood trauma, some personality disorders, inadequate family or peer support, being female, genetic vulnerability or psychiatric illness, recent stressful life changes, recent excessive alcohol intake and perception of an external locus of control. PTSD is usually accompanied with another axis 1 disorder. The most common comorbid disorders are; (a) alcohol abuse, (b) substance abuse (c) depression (d) anxiety disorders (especially phobia and panic disorder) (e) personality disorders (especially antisocial and borderline personality disorders). Some biological parameters are related with PTSD. Yetkin investigated 25 PTSD subjects at Gülhane Military Medical Academy Department of Psychiatry. PTSD patients had decreased sleep efficiency, decreased total sleep time, reduced stages 4 sleep, decreased REM latency, increased sleep latency and waking during sleep in comparison to the control subjects. Heart rate in sleep stages were significantly elevated In PTSD subjects and there were not any difference between stages. The number of brain imaging studies in PTSD has increased in the last years. Studies can be classified as structural, functional and receptor imaging studies. The structurel pathological main sites are hippocampus, parahipocampal gyrus, and amygdale. Functional imaging studies revealed that hyperactive or altered function of emotion response brain regions, such as amygdala and the insula, and hypofunction of emotion-regulatory structures, such as the medial prefrontral and cingulate cortex. Brain imaging neurochemical receptors have suggested that γ-aminobutyric acidergic and μ-opioid mechanisms may underlie brain activation patterns found in functional studies of PTSD. In a PTSD study in 1998 Doruk has investigated 48 PTSD subjects. 40 of those patients did have PTSD due terrorism. There were negative correlation between serum cortisol levels and intrusive-avoidance symptoms, between serum cortisol/noradrenalin and global severity, and between serum GH levels and avoidance-hyperarousal symptoms. However, between 24 hour urine cortisol/adrenaline and intrusive-hyperarousal symptoms had positive correlation.